An electrically compensated radiometer

Abstract

The radiometer described applies electronic feedback to dynamically balance the radiant power absorbed by a radiation absorbing sensor through an equivalent electric joule heating of a compensating sensor shielded from incident radiation. The instrument combines simple construction and the small time constant achieved in thermocouple detectors with the self-calibration and large output signal characteristic of resistance thermometer instruments. Theoretical instrument response equations indicate that, for perfect compensation, with identical sensor geometries, orientations and materials, output is independent of ambient temperature or sensor inclination. Measurement stability is determined principally by feedback amplifier drift. Initial measurements on a simple model pyranometer confirm the validity of the concept. For 3500 readings with radiant flux in the range of 100 to 800 W/m 2 and incidence within about 40° from the vertical, the standard deviation of the model was 0.4% and the linear correlation coefficient 0.988 with respect to an Eppley PSP pyranometer. Threshold radiant flux was about 100 ± 8 mW/m 2 and the expected uncertainty at a radiant flux of 1 kW/m 2 about 2 W/m 2. The time constant, for 20 μm tungsten wire sensors was of the order of 45 ms.